Abstract

Repeated thermal shock cracking is common in the operation of pressure equipment where water and steam are present. Surprisingly, it is not fully covered in the ASME Boiler and Pressure Vessel code nor in fitness-for-purpose recommended practice such as API 579. An example of thermal shock stresses occurs when hot surfaces are exposed to splashing of cold water. This eventually may lead to crack nucleation and crack growth. However, not all thermal shock cracks lead to failures (such as rupture, leak, or, in more brittle material, fragmentation), indeed the most frequent situation is that the cracking arrests at a depth of a few millimeters. This paper presents a unique experimental study and analysis of the information being gained from this study in terms of design guidelines and crack growth mechanisms. In the experiments, cracks are initiated and then grown in low carbon steel specimens exposed to repeated thermal shock. The test-rigs achieve large thermal shocks through the repeated water quenching of heated flat plate specimens. The effect of steady state loads on the growth and environmental effects due to the aqueous nature of the testing environment are found to be major contributors to the crack growth kinetics. The most important findings are that the conditions leading to both the initiation and the arrest of cracks can be identified and that the depth of a starter notch contributes little to the crack propagation.

(a) Cracking from external corners such as at penetrations. (b) Cracking at internal corners. (c) Examples of geometrical details affecting thermal shock cracking from power stations. Left at the intersection of two drain lines. Right in an economizer inlet header.

Crack growth rate versus crack length (includes notch depth) for a number of cracks in the vertical furnace experiments. T=maximum cycle temperature (°C), P=primary mechanical load (MPa), Q=quenching time (time of water application) in seconds. Starting notch depth, ao, is 3.5mm for all cases. The meaning of “Best fit” and “Conservative” lines are described in relation to Eq. 3.

Two cases differing only in applied mechanical loading. The applied 90MPa loading shows a higher level of HSF growth and an area of corrosion dominated growth. A picture of this crack is shown in Fig. 7. Data drawn from Fig. 5, 7s quench.

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